In current semiconductor processing technology, lead/tin solders are used to couple the metal interconnect layers of a semiconductor chip to, for example, the leads of a chip cafrier package. In this technology, a pad limiting metallurgy ("PLM") consisting of a plurality of metal layers (typically chromium, copper and gold layers) is used to provide a wetting surface for the solder during reflow while also insuring ohmic contact to the underlying metal layers. It is particularly important for the PLM to be mechanically sturdy, and to have a low contact resistance. A low PLM to metal contact resistance using so-called "wet cleaning" techniques (e.g. a chemical solution of phosphoric acid, chromium trioxide and water) has been obtained using a barrier layer of pure titanium in the PLM. However, the adhesion of the titanium PLM to the underlying organic layer (as measured by an Instron pull test measurement tool) decreases to unacceptable levels upon subsequent thermal cycling, (e.g., during the course of joining the deposited solder bumps to a chip carrier). This decrease in adhesion becomes even more pronounced after several chip join cycles. In general, the decrease in adhesion is believed to be caused by a titanium-polyimide chemical reaction. The article entitled "Mechanical and Surface Analytical studies of Titanium/Polyimide Adhesion," Proceedings of the Symposium on Multilevel Metallization, Interconnection, and Contact Technologies, Volume 87-4, pgs. 144-149, by Furman et al, discusses the titanium-polyimide adhesion problem in detail.
This adhesion failure is absent when a pure chromium barrier layer is formed in the PLM. However, the sputter cleaning techniques (e.g. gas plasma) required to obtain low chromium PLM to metal contact resistance will cause charge accumulation on the underlying metal layer, thereby degrading the performance of the active and/or passive devices interconnected by the metal layer. Moreover, sputter clean cycles are exceedingly slow, and as such are not conducive to volume manufacturing applications.
The following references are examples of the state of the PLM fabrication art where it is generally known to form a barrier layer in the PLM from chromium-based alloys.
U.S. Pat. No. 3,567,508 to Cox et al discloses a method of forming a metallic electrical contact on a semiconductor body wherein a barrier layer consists of a host of active metals including titanium, vanadium, chromium, niobium, zirconium, palladium, tantalum and intermetallic compounds thereof.
U.S. Pat. No. 4,231,058 to Gleason discloses a metallization layer for a trapped plasma avalanche triggered transit (TRAPATT) diode wherein the metallization layer consists of 56% W, 24% Ti and 20% Cr.
U.S. Pat. No. 4,268,849 to Gray et al discloses a raised bonding pad structure in which the barrier layer of the structure consists of a nickel-chromium alloy layer. The alloy layer may have varying amounts of nickel up to 80% by weight.
U.S. Pat. No. 4,463,059 to Bhattacharyya et al discloses a layered metal film structure for LSI chip carriers. The structure includes a thin adhesion layer consisting of Cr, Ti or other group IVB, VB or VIB metals. The top surface metallurgy of the structure includes layers of Cu, Cr and Au.
U.S. Pat. No. 4,164,607 to Thiel et al discloses a thin film resistor including an alloy layer, consisting of nickel, chromium and gold.
U.S. Pat. No. 3,959,047 to Alberts et al discloses a method of manufacturing semiconductor integrated circuitry. The method includes the step of depositing a composite metal film consisting of chromium, copper and gold.
The article entitled "Adhesion of Evaporated Titanium to Polyethylene: Effects of Ion Bombardment Pretreatment," J. Vac. Sci. Technol. A, pgs. 1498-1502, Vol. 2, No. 4., Oct.-Dec. 1984, by Bodo et al, discloses the effects on the adhesion of evaporated Ti to polyethylene (PE) when Ar+ is bombarded onto the surface of the PE prior to film deposition.
None of the previous or existing PLM structures providing both low contact resistance and high adhesion to underlying organic layers can be implemented with wet pre-cleaning techniques. As a result, there remains a continuing need in the art for a PLM structure which can be implemented with wet pre-cleaning techniques, and which exhibits both low contact resistance and optimized adhesion characteristics.